Question: Solve for $r$ : $18 = 30 + r$
Solution: Subtract $30$ from both sides: $ \begin{eqnarray} \\ \hphantom{18 {- 30}} &\hphantom{=}& \hphantom{r} \\ 18 &=& 30 + r \\ \\ {-30} && {-30} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 18 {- 30} &=& r \\ \end{eqnarray} $ Simplify. $ r = -12$